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• Building is nice - but what of physics?
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  The Gauss rifle project (it's not really a rifle, but never mind) is a pretty cool demonstration. However, the article is weak on what it's actually demonstrating. Yes, energy is transferred from one ball to the next. But, if that was the only thing going on, the last ball would leave the apparatus with a velocity identical to the velocity with which the first ball entered, much as what happens with a Newton's cradle. That's not what happens here, so something else is happening.
  
  The extra energy required to make the last ball go faster has to come from somewhere. The energy cannot come from the magnets themselves; magnets don't do that. You can't get energy out of a permanent magnet any more than you can get blood from a stone.
  
  It is useful here to compare the before and after states of the setup. Before it's fired, there are 4 balls up against a magnet and 4 more balls each 5/8" away from a magnet. After it's fired, all 8 balls are up against a magnet. Therein lies the answer.
  
  Before it's fired, the 4 "outer" balls have some potential energy. That is, if the "inner" balls weren't there, the outer balls would have the capacity to do work, to "fall" towards the magnet. They thus have potential energy precisely because they're being kept away from the magnet.
  
  After it's fired, these 4 "outer" balls are now touching a magnet; the key is to relize that it doesn't matter which magnet they're touching, just so long as they touch. Their potential energy therefore had to go somewhere. Turns out it went into the kinetic energy of the ball shooting out of the apparatus.
  
  Technically, this is a first law of thermodynamics issue, also known as the law of conservation of energy. It's even better known as, "there ain't no such thing as a free lunch."
  
  It is all good and well to build cool stuff; but, it's even cooler to be able to understand what it is that we build. I suggest that future physics demonstration articles in _Make_ should be accompanied by a decent explanation of the phenomena being demonstrated.

  Posted by davetyp on March 13, 2005 at 21:21:55 Pacific Time

• Building is nice - but what of physics?
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  Actually, the Gauss rifle works due to the pull of the magnets as the steel ball(s) approach them. This increase in velocity increases the kinetic energy of the steel balls.
  

  Posted by skiEurope on August 25, 2005 at 17:02:01 Pacific Time

• Building is nice - but what of physics?
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  davetyp, thank you for your explanation. I was just looking at the article, and I was also confused. I decided to check out the web site, and BINGO!!!

  Posted by alankilian on March 23, 2005 at 14:37:36 Pacific Time